L08 I. C. RUSSELL [JRFACE GEOLOGY OF ALASKA. 



. than the source of the " Tes-1 in -too " seems premature, as neither of these 

 streams has been fully explored, and their sources are unknown. 



In view of the facts jus! stated, it seems to me advisable to apply the name 

 Yukon to the main trunk of the drainage system, commonly known by that 

 name in the lower part of its course — that is, that the Yukon, the JYlly, the 

 Lewes below the mouth of the Tes-lin-too, ami the Tes-lin-too to its source, 

 a- designated by Dawson, he named the Yukon. 



Geologn \i. Structure of the Yukon Region. 



Monoclinals. — The prevailing trend of* the mountains and the strike of the 

 lock- throughout the Yukon region below the mouth of the Porcupine is, in 



teral, northeast and southwest. Along the Yukon, near the 141st meridian 

 and in the neighboring part of the North West Territory, the trend of the 

 main ranges is nearly east and west. Throughout the Yukon region in 

 Alaska the geological structure approaches that of the Great Basin of the 

 western United States. Nothing similar to the folds of the Appalachians or 

 the Alps has been observed in that region. The mountains are, in large 

 part, monoclinal ridges, hut do not reveal their structure as definitely as do 

 the ranges of Nevada and Utah. The presence of faults along the borders 

 of the upheaved orographic block can he readily determined, however, in 

 many instances. 



Faults. — The finest example of monoclinal structure seen in ascending the 

 Yukon, though on a comparatively small scale, was in cliffs of sandstone and 

 Blate bordering the light hank of the river for several miles, at a locality 

 some fifteen or twenty miles below the mouth of the Meloikakat, or midway 

 between Nulato and Nowikakat. These sandstones contain the haves of 

 deciduous tree- and belong to the same system as the rocks at Nulato, which 

 have been di scribed by Dall.* 



The river hank at the locality referred to is extremely precipitous, and 

 ezp08( - a fine Bection of the rocks, which dip. in general, northwest 25° to ''0°, 

 • jit where disturbed by faults. The displacement- trend nearly moth 

 and ,-oiith and appear in the cliffs as in a diagram. In the he.-t exposed 

 portion of the section there are six or eight important faults within a .-pace 

 ofaboui two mile.-. These are parallel and head to the easl at angle- ranging 



from 25 t" I" . Iii each instance the strata are disturbed on approaching 



the breaks, but BOOH return to their normal dip. At each fault a lateral 



valhy ha- been excavated, the wesl side of which is a smooth, even, rock 

 -lope, frequently Blickensided, ami is in reality the heaved Bide of the fault. 

 The easl wall of each of the valhy- i- rugged and broken, and the Btrata in 

 the projecting ledges usually show a high dip towards the east. In other 



\ ol. 16, 1868, pp. B7 



